PROJECT SUMMARY Despite persistent overweight/obesity (OW/O) disparities by socioeconomic status (SES), interventions targeting preschoolers from low-SES backgrounds are sparse. Based on the intergenerational transmission of habits and obesity, targeting preschoolers and their caregivers (parents or legal guardians) simultaneously is a promising strategy for the prevention of OW/O. This project will determine the preliminary efficacy of an innovative intergenerational intervention among Head Start preschoolers, aged 3-5 years, and their caregivers. Methods: A two-group cluster randomized controlled trial will be conducted. Six Head Start centers will be randomly assigned to the intervention (n=3) or control group (n=3), and 24 caregiver-preschooler dyads will be recruited from each center (N=144 dyads). Grounded in an Actor-Partner Interdependence Model (APIM), the 16-week intervention has 3 components: 1) a caregiver component, including 1a) a Facebook-based program with weekly electronic retrievable flyers providing health information and behavioral change strategies and 4 weekly habit-formation tasks to improve parenting practices and home environment for preschoolers; and 1b) 3 face-to-face meetings (wks. 1, 8, & 16) to establish personal connections and communication networks among caregivers, discuss strategies, and share community resources to support preschoolers? behavioral changes at home; 2) a caregiver-preschooler learning component via Facebook messenger to send preschooler letters to each caregiver privately by the research team twice per week to 2a) share the preschooler?s experiences of learning at school and his/her interests for a healthy diet and physical activity at home, and 2b) elicit caregivers? response to the letters; and 3) a Head Start center-based preschooler component to help preschoolers establish healthy habits via weekly healthy diet and physical activity participatory learning. AIM 1: Determine the preliminary efficacy of intervention vs control on preschoolers? proximal behavioral (e.g., MVPA, diet quality) and distal anthropometric outcomes (e.g., proportion of OW/O, BMI z-score). AIM 2: Examine the preliminary efficacy of intervention vs control on caregivers? behavioral and anthropometric outcomes. AIM 3: Compare intervention vs control on the bidirectional relationship (proposed in APIM) between preschoolers and caregivers on MVPA, diet quality, and screen time. Innovation: The intervention extends beyond prior research that focuses only on the unidirectional influence of caregivers on preschoolers with a bidirectional approach that also emphasizes the influence of preschoolers on caregivers. The potential for sustainability and scalability is high because the intervention is integrated into daily routines and capitalizes on the already- existing social network Facebook to connect caregivers to an online private group. It facilitates the communication of preschooler preferences to improve caregivers? parenting practices through preschooler letters sent to caregivers via Facebook messenger. Impact: The proposed study will set the stage for a future large-scale study to prevent and reduce OW/O and promote health among underserved preschoolers.